1. Technical Field of the Invention
The present invention relates to a vacuum pump used in a semiconductor manufacturing process or the like. More specifically, the present invention relates to a vacuum pump having a screw groove formed:
either on the inside periphery of a cylindrical stator;
or on the outside periphery of a rotor disposed in the interior of the stator; wherein
gas flowing between the stator and the rotor is compressed by rotating the rotor at a high speed.
Examples of the aforementioned vacuum pump having a screw groove include: a screw type vacuum pump; and a multiple-type vacuum pump which is a combination of a screw type vacuum pump and a turbo-molecular pump: both of which have heretofore been used.
2. Prior Art
In a semiconductor manufacturing process, a vacuum pump is used for the purpose of keeping a reaction chamber in a high vacuum and of discharging reaction byproducts. A vacuum pump used for the aforementioned purpose is required to be capable of being used in a wide pressure range.
One known vacuum pump capable of being used in a wide pressure range as mentioned above is a screw type vacuum pump. In the case of a screw type vacuum pump, a screw groove is formed either on the inside periphery of a cylindrical stator or on the outside periphery of a rotor disposed in the interior of the stator, and the rotor is rotated at a speed as high as tens of thousands of revolutions per minute. The rotor is disposed such that the pertinent clearance between the rotor and the stator is 0.2 mm to 1.0 mm, or so, which is extremely small.
With such a screw type vacuum pump, gas is caused to flow in the direction of the axis along the screw groove between the rotor and the stator, owing to the viscosity of gas, resulting in gas being compressed.
Such a screw type vacuum pump has an excellent gas exhaustion capacity, particularly in a low-vacuum range (a vacuum range close to atmospheric pressure). In a high-vacuum range, however, it is often the case that a multiple-type vacuum pump, which is a combination of a screw type vacuum pump and a turbo-molecular pump, is used, since the gas exhaustion speed is somewhat low in the aforementioned range.
Incidentally, with a semiconductor manufacturing process, it is often the case that foreign matter such as powder of reaction byproducts is included in the gas exhausted from a reaction chamber, and in the event that a vacuum pump is used, the aforementioned foreign matter is liable to stick to, and to deposit on, the inner surface of the vacuum pump. In particular, in the case of a screw type vacuum pump, foreign matter such as powder is liable to deposit in the clearance between then rotor and the stator, since the aforementioned clearance is designed to be extremely small as mentioned above. Therefore in the event that the stator is fixed, there is a danger that the rotor, when rotating at a high speed, engages with the stator, with foreign matter interposed in-between, resulting in instantaneous locking. Such instantaneous locking of the rotor causes the rotor and the stator to be damaged.
Such being the case, an arrangement has been contrived wherein the stator is supported in such a way as to permit rotation with respect to the housing, thereby allowing the stator to rotate together with the rotor upon engagement of the rotor with the stator, with the result that the rotational speed of the rotor decreases gradually owing to the rotational resistance of the stator (refer to JPN. U.M. Appln.Koukoku 6-40954 for an example).
As regards a prior art screw type vacuum pump, even in a case where the stator is permitted to rotate as mentioned above, the degree of fit pertaining to the stator and the housing is such that the outside periphery of the stator and the inside periphery of the housing are well-nigh in contact with each other when the aforementioned pump is used.
However, in a semiconductor manufacturing process, it is often the case that a corrosive gas is used, with the result that stress corrosion cracking or creep rupture occurs to the rotor. Cracks due to phenomena such as stress corrosion cracking occurs in the direction of the axis of the rotor. When such a crack occurs, the rotor becomes deformed during high-speed rotation such that its diameter expands owing to centrifugal force, thereby causing the rotor to come into contact with the stator, which is disposed around the rotor, with the result that the stator is deformed radially outward. This being so, in the event that the stator and the housing are well-nigh in sliding contact with each other, then the stator is pressed against the inner surface of the housing, thus making it impossible for the stator to rotate with respect to the housing, with the result that the rotor becomes locked instantaneously, leading to fracturing of the rotor owing to the resulting rotational inertia force. In the worst case, the stator becomes broken through, resulting further in failure of the housing as well.
In order for such failure of the housing to be prevented, the housing should be so designed as to have sufficient strength. However, for the purpose of doing so, it is necessary to enlarge the housing wall thickness, leading to a problem in that the weight of the vacuum pump increases.
In view of the aforementioned problem, it is an object of the present invention to obtain a light-weight vacuum pump which has a screw groove and whose housing does not fail even when the rotor becomes fractured during high-speed rotation.
For the purpose of attaining the aforementioned object, the present invention provides a vacuum pump (1, 101, or 201, as applicable) wherein a sufficiently large clearance (r) is provided between a cylindrical housing (2) and a stator (8 or 108 as applicable) which is supported inside the housing (2) in such a way as to permit rotation.
Namely, the vacuum pump (1, 101, or 201, as applicable) according to the present invention comprises: a cylindrical stator (8 or 108 as applicable) which is provided inside a housing (2) and which is supported in such a way as to permit circumferential sliding; and a rotor (22 or 122 as applicable) which is provided inside the stator (8 or 108 as applicable) in such a way as to permit free rotation and which is set in rotational motion by a motor (18+21): wherein a screw groove is formed either on the inside periphery of the stator (8 or 108 as applicable) or on the outside periphery of the rotor (22 or 122 as applicable); and a clearance (r) which is large enough to allow the stator (8 or 108 as applicable) to be deformed radially outward is provided between the stator (8 or 108 as applicable) and the housing (2).
Since a sufficiently large clearance is provided between the stator (8 or 108 as applicable) and the housing (2) as mentioned above, in the event that the rotor (22 or 122 as applicable) becomes fractured or deformed during high-speed rotation, causing part of the rotor (22 or 122 as applicable) to come into contact with the stator (8 or 108 as applicable), which in turn becomes deformed radially outward, even then the stator (8 or 108 as applicable) is prevented from coming into contact with the housing (2), thus permitting the stator (8 or 108 as applicable) to rotate, so long as the amount of radially outward deformation of the stator (8 or 108 as applicable) is smaller than the aforementioned clearance (r). Contacting of the rotor (22 or 122 as applicable) with the stator (8 or 108 as applicable) causes the stator (8 or 108 as applicable) to start rotating together with the rotor (22 or 122 as applicable), and the rotational speed of the rotor (22 or 122 as applicable) decreases gradually owing to the rotational friction of the stator (8 or 108 as applicable).
The longer the time is during which the aforementioned rotor (22 or 122 as applicable) and the aforementioned stator (8 or 108 as applicable) continue to rotate together with each other until the stator (8 or 108 as applicable) comes into contact with the housing (2) subsequent to the rotor (22 or 122 as applicable) hitting the stator (8 or 108 as applicable), the larger the amount of energy is which is absorbed owing to the rotational friction of the stator (8 or 108 as applicable) (or the larger the amount of rotational energy is which is converted into thermal energy).
As regards the aforementioned clearance (r), in the event that rxe2x89xa70.5 mm or so for example, even then the rotational speed can be reduced through the absorption of the rotational energy, of the aforementioned rotor (22 or 122 as applicable) and of the aforementioned stator (8 or 108 as applicable), caused by the rotational resistance of the stator (8 or 108 as applicable) which is provided until such time as the stator (8 or 108 as applicable) comes into contact with the housing (2). By letting rxe2x89xa71 mm, an increased amount of energy can be absorbed owing to the rotational resistance of the stator (8 or 108) which is provided until such time as the stator (8 or 108 as applicable) comes into contact with the housing (2).
Therefore the larger the aforementioned clearance (r) is, the longer the time is which is required for the aforementioned stator (8 or 108 as applicable) to come into contact with the aforementioned housing (2), resulting in all the larger amount of energy being absorbed owing to the aforementioned rotational resistance. The larger the amount of energy is which is absorbed as mentioned above, the lower the level is to which the rotational speed of the rotor (22 or 122 as applicable) and of the stator (8 or 108 as applicable) drops when the stator (8 or 108 as applicable) comes into contact with the housing (2), thereby increasing the effectiveness in preventing the housing (2) from being fractured. Accordingly by providing the aforementioned clearance (r), the rotor (22 or 122 as applicable) is prevented from being severely fractured, thus precluding the stator (8 or 108 as applicable) from being broken through and moreover preventing the phenomenon of even the housing (2) being fractured.